Reducing Source/Drain Resistance of III-V Based Transistors

ABSTRACT

An integrated circuit structure includes a substrate; a channel layer over the substrate, wherein the channel layer is formed of a first III-V compound semiconductor material; a highly doped semiconductor layer over the channel layer; a gate dielectric penetrating through and contacting a sidewall of the highly doped semiconductor layer; and a gate electrode on a bottom portion of the gate dielectric. The gate dielectric includes a sidewall portion on a sidewall of the gate electrode.

This application is a continuation of claims the benefit of U.S.Non-Provisional application Ser. No. 12/616,073, filed on Nov. 10, 2009,entitled “Reducing Source/Drain Resistance of III-V Based Transistors”which claims the benefit of U.S. Provisional Application No. 61/174,358filed on Apr. 30, 2009, entitled “Reducing Source/Drain Resistance ofIII-V Based Transistors,” both of which applications are herebyincorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to integrated circuit structures, andmore particularly to transistors comprising III-V compoundsemiconductors and methods for forming the same.

BACKGROUND

The speeds of metal-oxide-semiconductor (MOS) transistors are closelyrelated to the drive currents of the MOS transistors, which drivecurrents are further closely related to the mobility of charges. Forexample, NMOS transistors have high drive currents when the electronmobility in their channel regions is high, while PMOS transistors havehigh drive currents when the hole mobility in their channel regions ishigh.

Compound semiconductor materials of group III and group V elements(commonly known as III-V compound semiconductors) are good candidatesfor forming NMOS transistors due to their high electron mobility.Therefore, III-V compound semiconductors have been commonly used to formNMOS transistors. To reduce the manufacturing cost, methods for formingPMOS transistors using III-V compound semiconductors have also beenexplored. FIG. 1 illustrates a conventional transistor incorporatingIII-V compound semiconductors. In the formation process, a plurality oflayers is blanket formed on a silicon substrate, wherein the pluralityof layers includes a buffer layer formed of GaAs, a graded buffer layerformed of In_(x)Al_(1-x)As (with x between, but not equal to, 0 and 1),a bottom barrier layer formed of In_(0.52)Al_(0.48)As, a channel layerformed of In_(0.7)Ga_(0.3)As, a top barrier layer formed ofIn_(0.52)Al_(0.48)As, an etch stop layer formed of InP, and a contactlayer formed of In_(0.53)Ga_(0.47)As. A first etch is performed to etchthrough the contact layer (In_(0.53)Ga_(0.47)As) and stop at the etchstop layer (InP) to form a first recess. A second etch is then performedto etch through the etch stop layer (InP) and into a portion of the topbarrier layer (In_(0.52)Al_(0.48)As) to form a second recess. A gate,which is formed of metal, is then formed in the second recess. Theresulting transistor has the advantageous feature of a quantum wellformed of the bottom barrier layer, the channel layer, and the topbarrier layer.

The above-described structure and process steps, however, suffer fromdrawbacks. The contact layers (In_(0.53)Ga_(0.47)As) are horizontallyspaced apart from the gate by distance S. Further, the etch stop layer(InP) has a relatively wide bandgap and has a high resistivity.Therefore, there exists a high resistance path between the metalsource/drain and the channel layer. Therefore, the external resistanceof the source and drain regions is high, which adversely affects thedrive current of the transistor. A method and a structure for overcomingthe above-described shortcomings in the prior art are thus needed.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, an integratedcircuit structure includes a substrate; a channel layer over thesubstrate, wherein the channel layer is formed of a first III-V compoundsemiconductor material; a highly doped semiconductor layer over thechannel layer; a gate dielectric penetrating through and contacting asidewall of the highly doped semiconductor layer; and a gate electrodeon a bottom portion of the gate dielectric. The gate dielectric includesa sidewall portion on a sidewall of the gate electrode.

Other embodiments are also disclosed.

The advantageous features of the present invention include reducedsource and drain resistances and an increased drive current in theresulting transistor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a conventional transistor comprising III-V compoundsemiconductor materials of group III and group V elements; and

FIGS. 2 through 8 are cross-sectional views of intermediate stages inthe manufacturing of a transistor in accordance with an embodiment.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the embodiments of the present invention arediscussed in detail below. It should be appreciated, however, that theembodiments provide many applicable inventive concepts that can beembodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention.

Novel transistors comprising compound semiconductor materials of groupIII and group V elements (referred to as III-V compound semiconductorshereinafter) and the methods of forming the same are provided. Theintermediate stages in the manufacturing of embodiments of the presentinvention are illustrated. The variations of the embodiments arediscussed. Throughout the various views and illustrative embodiments ofthe present invention, like reference numbers are used to designate likeelements.

Referring to FIG. 2, substrate 20 is provided. Substrate 20 may be asemiconductor substrate formed of silicon, germanium, SiGe, InP, and/orother semiconductor materials. A plurality of layers, which may beformed of compound semiconductor materials, is epitaxially grown onsubstrate 20. In an embodiment, the plurality of layers includes bottombarrier layer 24, channel layer 26, and top barrier layer 28. In anembodiment, channel layer 26 has a first bandgap, while bottom barrierlayer 24 and top barrier layer 28 have a second bandgap greater than thefirst bandgap. Accordingly, layers 24, 26, and 28 form a quantum well.In an exemplary embodiment, the second bandgap is greater than the firstbandgap by about 0.1 eV, although greater or lower bandgap differencesmay also apply. The appropriate materials of channel layer 26, topbarrier layer 28 and bottom barrier layer 24 may be selected bycomparing the bandgaps of the available semiconductor materials withhigh carrier mobilities, which materials may include, but are notlimited to, silicon, germanium, GaAs, InP, GaN, InGaAs, InAs, InSb,InAlAs, GaSb, AlSb, AlAs, AlP, GaP, and combinations thereof. In anexemplary embodiment, channel layer 26 comprises In_(0.7)Ga_(0.3)As,while bottom barrier layer 24 and top barrier layer 28 compriseIn_(0.52)Al_(0.48)As. In other embodiments, channel layer 26 is formedof InGaAs, while bottom barrier layer 24 and top barrier layer 28 areformed of GaAs. In yet other embodiments, channel layer 26 is formed ofInAs, while bottom barrier layer 24 and top barrier layer 28 compriseInAlAs. Bottom barrier layer 24 may have a thickness between about 5 nmand about 10 μm, channel layer 26 may have a thickness between about 2nm and about 50 nm, and top barrier layer 28 may have a thicknessbetween about 5 nm and about 500 nm. It is realized, however, that thedimensions recited throughout the description are merely examples, andmay be changed if different formation technologies are used.

Optionally, additional buffer layer(s) such as buffer layer 22 may beformed on top of substrate 20. Buffer layer 22 may have a latticeconstant between the lattice constant of substrate 20 and the latticeconstant of the overlying layer such as bottom barrier layer 24, so thatthe transition of lattice constants from lower layers to upper layers isless abrupt.

FIG. 3 illustrates the formation of highly doped layer 30 on top barrierlayer 28. Highly doped layer 30 is formed of a semiconductor material,and may be in-situ doped to a high impurity concentration, for example,greater than about 1×10¹⁸/cm³, although a lower concentration can alsobe used. The impurity concentration in highly doped layer 30 may also begreater than the impurity concentrations in any of top barrier layer 28,channel layer 26, and bottom barrier layer 24. It is preferable to dopehighly doped layer 30 using in-situ doping instead of implanting, sothat the impurity introduced by the step of doping the highly dopedlayer 30 is not substantially introduced into top barrier layer 28. Theelement of the doped impurity is partially determined by thesemiconductor material of the highly doped layer 30. In an embodiment,highly doped layer 30 comprises silicon, germanium, carbon, and/orcombinations thereof. Accordingly, if the resulting transistor is anNMOS transistor, common n-type impurities such as phosphorous, arsenic,and combinations thereof may be used. Conversely, if the resultingtransistor is a PMOS transistor, the doped impurity may include boron.In other embodiments, highly doped layer 30 comprises III-V compoundsemiconductor materials such GaAs, InGaAs, InAs, InSb, GaSb, GaN, InP,and combinations thereof. Accordingly, if the resulting transistor is anNMOS transistor, the doped impurity may include silicon (Si).Conversely, if the resulting transistor is a PMOS transistor, the dopedimpurity may include zinc (Zn) and/or beryllium (Be). Highly doped layer30 may also have a bandgap smaller than that of top barrier layer 28. Asa result of the small bandgap and the high doping concentration, theresistivity of highly doped layer 30 is low. The formation methods ofhighly doped layer 30 include metal organic chemical vapor deposition(MOCVD), although other commonly used deposition methods can also beused.

Next, a gate-last approach is taken to form a gate structure, as isshown in FIGS. 4 through 7. FIG. 4 illustrates the formation of dummygate 32, gate spacers 36, and sacrificial inter-layer dielectric (ILD)38. Dummy gate 32 may be formed of polysilicon or other materials havinga high etching selectivity relative to gate spacers 36 and highly dopedlayer 30. Optionally, a dummy gate dielectric (not shown) may be formedbetween dummy gate 32 and highly doped layer 30. Gate spacers 36 may beformed of dielectric materials such as silicon oxide, silicon nitride,and composite layers thereof. The formation processes of dummy gate 32and gate spacers 36 are known in the art, and hence are not described indetail herein.

Sacrificial ILD 38 is then formed to a level higher than the top edge ofgate spacers 36. A planarization, for example, a chemical mechanicalpolish (CMP), is then performed. The planarization may stop at the topedge of gate spacers 36. As a result, dummy gate 32 is exposed, whilehighly doped layer 30 is covered.

Referring to FIG. 5, dummy gate 32 (and the dummy gate dielectric, ifany) is removed by etching, leaving opening 40, and hence the underlyinghighly doped layer 30 is exposed. Next, an additional etch is performedto remove the exposed portion of highly doped layer 30, with the etchstopping at top barrier layer 28. The etchant may be selected so thatthere is a high etching selectivity between highly doped layer 30 andtop barrier layer 28, and top barrier layer 28 is etched as little aspossible.

Referring to FIG. 6, gate dielectric layer 42 and gate electrode layer44 are formed to fill opening 40. Gate dielectric layer 42 may be formedof commonly used dielectric materials such as silicon oxide, siliconnitride, oxynitrides, multi-layers thereof, and combinations thereof.Gate dielectric layer 42 may also be formed of high-k dielectricmaterials. The exemplary high-k materials may have k values greater thanabout 4.0, or even greater than about 7.0, and may include aluminumoxide, hafnium oxide, hafnium oxynitride, hafnium silicate, zirconiumsilicate, yttrium oxide, cerium oxide, titanium oxide, tantalum oxide,and combinations thereof. Gate electrode layer 44 may be formed ofmetals such as TaN, TiN, Pd, Pt, Al, Au, Ni, Ti, Er, W and combinationsthereof, metal nitrides, metal silicides, doped polysilicon, and thelike.

A CMP is then performed to remove portions of gate dielectric layer 42and gate electrode layer 44 outside opening 40 (refer to FIG. 5). In theresulting structure, a gate structure including gate dielectric 50 andgate electrode 52 is left, as is shown in FIG. 7. Sacrificial ILD 38 isthen removed, so that highly doped layer 30 is exposed. It is noted thatgate dielectric 50 has a bottom portion contacting top barrier layer 28and has sidewall portions on the sidewalls of gate electrode 52. Thesidewall portions of gate dielectric 50 further separate gate electrode52 from gate spacers 36.

Next, as shown in FIG. 8, metal layer 54 is formed over highly dopedlayer 30, wherein metal layer 54 may include nickel, aluminum,palladium, gold, and/or the like. Additional anneal processes may beperformed so that metal layer 54 reacts with the underlyingsemiconductor layer, which may be highly doped layer 30 or an additionalcontact layer (not shown), to reduce the contact resistance. Throughoutthe description, metal layer 54 and the underlying highly doped layer 30are referred to as source and drain regions due to their relatively lowresistivities.

Optionally, additional contact layer(s) may be formed between metallayer 54 and highly doped layer 30, and may be formed of semiconductormaterials such as silicon, germanium, GaAs, InGaAs, InAs, InSb, GaSb,GaN, InP, and combinations thereof. The additional layer(s) maygenerally be allocated with the trend that the upper layers have higherdoping concentrations and/or lower bandgaps, while the lower layers havelower doping concentrations and/or higher bandgaps. Accordingly, theadditional contact layer(s) may have higher doping concentrations and/orlower bandgaps than highly doped layer 30. The elements of the dopedimpurities in the additional contact layer(s) may also be determinedbased on the materials of the additional contact layer(s), similar tothe relationship between highly doped layer 30 and the impuritiestherein. In alternative embodiments, the additional contact layer(s) andmetal layer 54 may be formed after the formation of gate spacers 36 andbefore the removal of dummy gate 32. Accordingly, sacrificial ILD 38 nolonger needs to be removed, and an additional ILD may be formed onsacrificial ILD 38.

The embodiments of the present invention have several advantageousfeatures. By forming the highly doped layer first and then adopting agate-last approach to form a gate structure extending into the highlydoped layer, a highly doped layer having a low resistance is close tothe gate structure. Further, the highly doped layer is formed directlyon the top barrier layer and hence there is no additional highresistance etch stop layer therebetween. Therefore, the source/drainresistance is small, and the drive current of the resulting transistoris high.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, andcomposition of matter, means, methods and steps described in thespecification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps. In addition, eachclaim constitutes a separate embodiment, and the combination of variousclaims and embodiments are within the scope of the invention.

What is claimed is:
 1. A device comprising: a channel layer over asubstrate, wherein the channel layer is formed of a first III-V compoundsemiconductor material comprising group III and group V elements; a topbarrier over the channel layer, a bandgap of the top barrier greaterthan a bandgap of the channel layer; a doped semiconductor layer overthe top barrier; a gate dielectric extending through the dopedsemiconductor layer; and a gate electrode on a bottom portion of thegate dielectric, wherein the gate dielectric comprises a sidewallportion on a sidewall of the gate electrode.
 2. The device of claim 1further comprising a bottom barrier over the substrate, and whereinforming the channel layer comprises forming the channel layer over thebottom barrier.
 3. The device of claim 2, wherein a bandgap of thebottom barrier is greater than a bandgap of the channel layer.
 4. Thedevice of claim 1, wherein the gate dielectric is in contact with asidewall of the doped semiconductor layer.
 5. The device of claim 4,wherein a bottom surface of a bottom portion of the gate dielectric issubstantially even with a bottom surface of the doped semiconductorlayer.
 6. The device of claim 1, wherein the doped semiconductor layeris doped with an impurity having a concentration greater than about1×10¹⁸/cm³.
 7. The device of claim 6, wherein the doped semiconductorlayer comprises a second III-V compound semiconductor material, andwherein the impurity is selected from the group consisting essentiallyof Si, Zn, Be, and combinations thereof.
 8. The device of claim 7,wherein the second III-V compound semiconductor material is selectedfrom the group consisting essentially of GaAs, InGaAs, InAs, InSb, GaSb,GaN, InP, and combinations thereof.
 9. The device of claim 8, whereinthe first III-V compound semiconductor material is selected from thegroup consisting essentially of GaAs, InGaAs, InAs, InSb, GaSb, GaN,InP, and combinations thereof.
 10. A method of forming a devicecomprising: forming a channel layer over a substrate, wherein thechannel layer is formed of a first III-V compound semiconductor materialcomprising group III and group V elements; forming a top barrier overthe channel layer, a bandgap of the top barrier greater than a bandgapof the channel layer; forming a doped semiconductor layer over the topbarrier; forming a gate dielectric extending through the dopedsemiconductor layer; and forming a gate electrode on the gatedielectric, wherein the gate dielectric comprises a sidewall portion ona sidewall of the gate electrode.
 11. The method of claim 10, furthercomprising etching a recess extending through the doped semiconductorlayer, and wherein the forming the gate dielectric comprises forming thegate dielectric in the recess.
 12. The method of claim 11, wherein theforming the gate dielectric comprises forming the gate dielectric on asidewall of the doped semiconductor layer.
 13. The method of claim 10,further comprising forming a bottom barrier over the substrate and belowthe channel layer, wherein a bandgap of the bottom barrier is greaterthan a bandgap of the channel layer.
 14. The method of claim 10, whereina bottom surface of a bottom portion of the gate dielectric issubstantially planar with a bottom surface of the doped semiconductorlayer.
 15. The method of claim 10, wherein the doped semiconductor layeris doped with an impurity having a concentration greater than about1×10¹⁸/cm³.
 16. A method of forming a device, comprising: forming abottom barrier over a substrate; forming a channel layer over the bottombarrier, wherein the channel layer is formed of a first III-V compoundsemiconductor material comprising group III and group V elements, andwherein a bandgap of the bottom barrier is greater than a bandgap of thechannel layer; forming a doped semiconductor layer over the channellayer; forming a gate dielectric penetrating through and contacting asidewall of the doped semiconductor layer; and forming a gate electrodeon the gate dielectric, wherein the gate dielectric comprises a sidewallportion on a sidewall of the gate electrode.
 17. The method of claim 16,further comprising forming a top barrier over the channel layer, andwherein the forming the doped semiconductor layer comprises forming thedoped semiconductor layer over the top barrier.
 18. The method of claim16, wherein the highly doped semiconductor layer comprises asemiconductor material selected from the group consisting essentially ofsilicon, germanium, carbon, and combinations thereof.
 19. The method ofclaim 16, wherein the doped semiconductor layer comprises a second III-Vcompound semiconductor material, and wherein the doped semiconductorlayer is doped with an impurity selected from the group consistingessentially of Si, Zn, Be, and combinations thereof.
 20. The method ofclaim 19, wherein the second III-V compound semiconductor material isselected from the group consisting essentially of GaAs, InGaAs, InAs,InSb, GaSb, GaN, InP, and combinations thereof.